1. FIELD OF THE INVENTION
The present invention relates to a direct casting process for manufacturing a photochromic article, in particular a photochromic ophthalmic lens.
2. DESCRIPTION OF THE PRIOR ART
It is known to use a variety of spiro-oxazine compounds as materials which impart photochromic properties to a polymerised host material such as a polymer of a polyol (allyl carbonate) monomer. Photochromism is a physical phenomenon which occurs when a chemical compound changes colour when it is exposed to ultraviolet radiation and then returns to its original colour when it is removed from the influence of the radiation.
Certain spiro-oxazine compounds have been suggested for use in making photochromic sunglasses and opthalmic lenses from synthetic plastic host materials. Examples of suitable spiro-oxazine photochromic compounds are given in U.S. Pat. No. 4215010, GB patent application No. 2117390A, European patent application No. 141407 and International patent application No. 85/02619.
A number of different ways of applying the photochromic compound or incorporating it into the host material have been described in the prior art. Typically, such methods include dissolving or dispersing the photochromic compound in the host material. One way of dispersing the photochromic compound into the host material is by "imbibation", i.e. diffusion of the photochromic compound into the host material by a suitable transfer mechanism such as immersion, thermal transfer, or vapour phase transfer.
European patent application No. 14107 suggests that photochromic spiro-oxazine compounds or compositions can be mixed with a polymerisable composition and cured to form an optically clear film, sheet or lens having photochromic properties. However, we have found that such a direct casting method does not readily produce satisfactory ophthalmic lenses having the required photochromic properties.
It appears that the free-radical polymerisation initiators which are conventionally used to initiate polymerisation of the monomer material used to form the synthetic plastic host material degrade the spiro-oxazine photochromic compounds.
Thus, for example, we have found that when one attempts to polymerise a monomer used very commonly in this field (diethylene glycol bis allylcarbonate--widely referred to by its brand name CR39) using a normal concentration of catalyst of about 3% by weight, and follows the procedure suggested in European patent application No. 141407--i.e. mixing the spiro-oxazine compound into the polymerisable composition and then curing the mixture--one finds that degradation of the spiro-oxazine compound is very extensive and no useful photochromic properties are exhibited by the resultant product.